Contacting hydrogen cyanide with a liquid hydroxide solution produces a cyanide solution according to the following chemical reactions: EQU MOH+HCNMCN+H.sub.2 O (1)
Where M is a metal cation, such as an alkali metal. In most commercial applications, M is sodium.
Sodium cyanide is commercially available in two forms: a 30 wt. % aqueous solution and 95 to 99 wt. % solid briquettes and granules. The predominant use for sodium cyanide is as a reagent in the isolation of precious minerals, particularly gold. Minor quantities are used in electroplating and in chemical synthesis.
There are two general types of sodium cyanide producers: those that manufacture solid sodium cyanide and market the product in a wide geographical area, including exports, and those that produce sodium cyanide solution and market the product locally, usually within a radius of about 250 miles of its production point. Because of solubility limitations, stable sodium cyanide solutions are limited to concentrations around 30% by weight. Since the mining industry uses sodium cyanide solution in its operations, this product form is usually preferred.
The hydrogen cyanide which is used to produce cyanide salts is produced from several processes. The most common process involves the reaction of natural gas with ammonia, in the presence of oxygen or air, over precious metal catalysts. This reaction is known as the Andrussow process. The products of this reaction are a diluted stream of hydrogen cyanide, water, hydrogen, carbon dioxide, carbon monoxide, as well as the excess quantities of ammonia and methane, along with nitrogen which came with the air source. Other minor contaminants may include other nitrites formed in the reaction. In most processes, this gas stream is purified by isolating the hydrogen cyanide to a purity of at least 99.5%.
A second reaction involving natural gas and ammonia was developed by Degussa and is known as the BMA process. The natural gas and ammonia are reacted over a precious metal catalyst, but in the absence of oxygen. The products of this reaction are similar to those of the Andrussow process, except that there is no water, carbon dioxide or carbon monoxide. The hydrogen cyanide is normally purified as anhydrous hydrogen cyanide through distillation before it is reacted to form other cyanide derivatives or metal cyanides.
In a third process, hydrogen cyanide can be prepared as a bi- or co-product from the production of acrylonitrile in which propylene and ammonia are reacted in the presence of air to yield acrylonitrile. Approximately one pound of hydrogen cyanide is produced for every 10 or 11 pounds of acrylonitrile produced. It is isolated as 99.5% anhydrous hydrogen cyanide before being reacted with caustic or other materials.
In the world today, DuPont produces approximately 25% of all sodium cyanide manufactured. Degussa produces about 20%, and ICI produces about 15%. DuPont uses the Andrussow process and isolates the hydrogen cyanide before manufacture of sodium cyanide. DuPont also produces some sodium cyanide from the co-product hydrogen cyanide production from acrylonitrile. Degussa uses the BMA process and isolates/purifies the hydrogen cyanide before manufacture of sodium cyanide. ICI uses the Andrussow process and isolates/purifies the hydrogen cyanide stream before manufacturing sodium cyanide.
Besides using pure hydrogen cyanide, most producers also use high purity sodium hydroxide. The purity of these raw materials has been found to be necessary to obtain metal cyanide salt having a desirable crystal size and crystal habit.
The aqueous metal cyanide solution is typically concentrated at higher temperature and under reduced pressure by evaporation of water, as disclosed in U.S. Pat. No. 3,619,132. Metal cyanide salt precipitates as a fine-crystalline paste, which is separated, dried, and further processed to obtain the final commercial forms: powder, granules, or briquettes. The drying and product handling equipment must include expensive filters, scrubbers, seals, and other equipment to contain any cyanide dust, which is extremely toxic. These cyanide treatment costs for water and air represent a substantial capital expense.
Producing metal cyanide salt granules and briquettes through compacting requires a specific moisture content and composition. For example, solid sodium cyanide is easily compactable with a water content between 0.2 and 1% by weight. If the moisture content is too low, the compacted product disintegrates. If the moisture content is too great, the compactor becomes clogged. In practice, it is difficult to obtain the required moisture content due to impurities in the raw materials and water. In addition, other common impurities, such as sodium formate, sodium chloride, sodium hydroxide, iron and other impurities can impede formation of large crystals and negatively affect compacted product formation.
Dry alkali cyanide is shipped to consumers in specially designed containers which exclude exposure to atmospheric air. Anhydrous sodium cyanide is hygroscopic, and it can absorb substantial quantities of water causing serious shipping and storage problems due to caking.
Many consumers of alkali cyanide dissolve the dry product in water. To avoid hydrolysis and the evolution of harmful hydrogen cyanide vapors according to reaction (1) above, a metal hydroxide salt, such as sodium or potassium hydroxide, is added to the dilution water.
Another potentially dangerous problem when handling dry alkali cyanide is dust formation. Because of the extreme toxicity of metal cyanide salts, appropriate containment of metal cyanide dust is necessary, adding to the ultimate cost of manufacturing and using dry metal cyanide salts.
One method of overcoming the disadvantages of drying cyanide salts to produce powder, granules or briquettes is to not crystallize, separate, and dry the cyanide salt, but to simply transport the metal cyanide as a concentrated solution, typically from about 30 wt. % to 40 wt. % solution. This method avoids the expense of crystallizing and drying. It also lessens the consumers' expense of forming a usable metal cyanide solution at a manufacturing or mining site. However, the transportation expenses are significantly higher because about 70% of the product shipped is water.
Other attempts have been made to overcome the disadvantages of drying alkali cyanide. U.S. Pat. No. 4,902,301 describes a process for preparing and delivering of solutions of sodium cyanide purposefully engineered to congeal in the dihydrate state of the product. The dihydrate product is prepared by evaporatively cooling a sodium cyanide slurry at a concentration greater than 58 wt. %, and preferably from 60 to 65 wt. %.
According to U.S. Pat. No. 4,902,301, the sodium cyanide slurry contains 26 percent solid crystals, but may contain up to 40 percent solid crystals. The solid crystals are not separated from their mother liquor. Instead, the entire slurry is quickly solidified to minimize settling or separation of the solid sodium cyanide crystals. The final product contains approximately 65 to 80 wt. % dihydrate and about 15 to 30 wt. % anhydrous sodium cyanide crystals. The cost of shipping remains high because of the large quantity of water (35 wt. % to 40 wt. %) remaining in the final product.
There is, therefore, a need in the art for an improved method for producing and shipping metal cyanide salts which does not require expensive drying equipment, filters, seals and other devices needed to handle metal cyanide dust.
It would also be an advancement in the art to provide a method for producing and shipping metal cyanide salts which avoids the expense and problems associated with drying, but which enables very high concentrations of metal cyanide to be shipped, thereby reducing transportation costs.
It would be a further advancement in the art to provide a method for producing and shipping metal cyanide salts which can be used with impure raw materials and process streams.
Such methods for producing and shipping metal cyanide salts are disclosed and claimed herein.